This is the kind of attitude that greatly limits the playing time of quarterbacks. If someone can play RT, then he can play RG. If he can play LT, he can play LG. Since LT's are the cream of the top, they can usually play RT.
HOWEVER, LT's are NOT, are NOT easy to find. Just converting RT's doesn't work.
That being said, Scar has worked with Cannon for many years, and Cannon may be ready to play LT.
There's no attitude involved. It's a fact. You can put together a decent OL without having a "true" LT. It changes a lot of things, sure, but it's doable. My point is that, if it were really necessary to have a premium guy at LT to be successful or keep your QB upright with reasonable consistency, about 25 of the 32 teams would have no prayer of putting out a decent offense from season to season.
Again, this is the trend on the OL in the NFL. Cross training is how teams keep it together over the course of a season. If you look at the snap counts of OL on average, most teams rarely get the type of consistency you would ideally like to have from their five best OL.
Teams are also in the midst of a "best five" era in terms of management of personnel. It's necessary, because of the relative dearth of what would be considered to be prototypical OL prospects.
The Eagles, for example, used nine OL in 2017. You really can't look at the OL as position specific right now and hope to be consistently successful, unless you get incredibly good luck regarding injuries and other attrition.
In fact, you contradicted your own point by making the very correct observation that it's really difficult to find those prototypical left tackles.
So, reasonably speaking, if there are, say, five of those guys in the league at any given time (or you can make it ten, it's the same idea), obviously most teams aren't going to have one, plus, since they're at a premium, getting one is going to be expensive and impact what you can do at other positions.
Yet teams succeed without that luxury all the time. It's a trend in the NFL.